1. Field of the Invention
This invention relates generally to amplification circuits and more particularly to a circuit and a method for controlling the relative gains of a first and second amplifier.
2. Description of the Prior Art
In audio systems having two or more channels, some means for adjusting the relative gains of the channels is required. This means is commonly referred to as a balance control. A desirable feature of such an audio system is to maintain the sum of the power gains provided by the channels substantially constant while varying the relative gains of the channels since this provides a substantially constant total audio output power. Existing balance systems do not provide true constant power gain. Although attempts have been made in the past to achieve a constant power gain balance control mechanically via shaped potentiometer laws, no known system has achieved this result electronically.
A stereo control system employing variable gain amplifiers has been disclosed in copending application "Improved Remote Control Amplifier" by Gay, Ser. No. 776,332, now U.S. Pat. No. 4,070,633, "Remote Controlled Amplifier" invented by Gay which is assigned to the assignee of the present invention and which is hereby incorporated by reference. The variable gain amplifiers disclosed in this reference have a gain which is exonentially dependent upon an applied control voltage. The reference disclosed circuitry for adjusting the volume and balance to two such variable gain amplifiers in a stereo control system. The use of variable gain amplifiers is desirable because the audio signal received by the amplifier need not pass through a control potentiometer in order to vary the magnitude of the amplified output signal, thereby avoiding radiation problems which normally result when wires are coupled from the control potentiometer to the signal processing circuit. Instead, the gain is controlled by a d.c. (direct current) voltage which may be generated in a location remote from the variable gain amplifiers. However the prior art does not disclose a circuit or a method for varying the relative gains of two remote controlled amplifiers in such a way that the total power gain remains constant over said variation.